The "snd_pcm_substream" handle was not initialized properly
in hda-loader.c for firmware load.

When the HDA DMAs were used to load the firmware,
the interrupts related to firmware load also triggered
calls to snd_sof_pcm_period_elapsed() on a non-existent ALSA
PCM stream.

This caused runtime kernel warnings from
pcm_lib.c:snd_pcm_period_elapsed().
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20190927200538.660-11-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>